1. Field of the Invention
The present invention relates to a positive photosensitive lithographic printing plate which can be exposed by near infrared rays, a method for producing it and a method for forming a positive image employing it. More particularly, it relates to a positive photosensitive lithographic printing plate suitable for direct plate making by means of a semiconductor laser or a YAG laser, a method for producing it and a method for exposing and developing it.
2. Discussion of the Background
Along with the progress in the image treating technology by computers, attention has been drawn to direct system for making a positive photosensitive or heat sensitive plate wherein a resist image is formed directly from digital image information by e.g. a laser beam or a thermal head without resort to a silver salt masking film.
Particularly, it has been strongly desired to realize a laser photosensitive direct plate making system with a high degree of development, employing a high power semiconductor laser, from the viewpoint of the long useful life of the semiconductor laser and downsizing.
An image-forming method utilizing laser photosensitivity or heat sensitivity is known for preparing a lithographic printing plate utilizing a sublimable transfer dye. For example, a method of preparing a lithographic printing plate by utilizing the crosslinking reaction of a diazo compound (e.g. JP-A-52-151024, JP-B-2-51732, JP-A-50-15603, JP-B-3-34051, JP-B-61-21831, JP-B-60-12939 and U.S. Pat. No. 3,664,737), or a method of preparing a lithographic printing plate by utilizing the decomposition reaction of nitrocellulose (e.g. JP-A-50-102403 and JP-A-50-102401), has been known.
Further, in recent years, a technique in which a chemical amplification type photoresist is combined with a long wavelength light ray absorbing dye, has been proposed. For example, JP-A-6-43633 discloses a photosensitive material wherein a certain specific squarilium dye is combined with a photo-acid-generator and a binder.
A technique of this type has been proposed for preparing a lithographic printing plate by exposing a photosensitive layer containing an infrared ray absorbing dye, a latent Brxc3x8nsted acid, a resol resin and a novolak resin, in an image pattern by e.g. a semiconductor laser (JP-A-7-20629). Further, the same technique wherein an s-triazine compound is used instead of the above latent Brxc3x8nsted acid, has also been disclosed (JP-A-7-271029).
However, these conventional methods were not necessarily adequate in their performance from a practical viewpoint. It was usually necessary to have a heat treatment step after the exposure phase of the plate making process. This extra heat treatment step resulted in increased operation time, the requirement of extra space for apparatus and higher equipment costs. Further, because the temperature in the heating step significantly affects the sensitivity, printing resistance and chemical resistance, users of the process were required to strictly control the temperature.
To overcome such problems, a positive photosensitive lithographic printing plate of the thermal conversion type, which requires no heat treatment, has been proposed in JP-A-8-207013. A further technique has been disclosed in which there is formed a photosensitive layer insoluble in an alkali developer. The layer is comprised of a novolak resin, a cyanine dye and a solubility-suppressing agent such as a sulfonic ester. The layer is irradiated with a laser so that the photosensitive layer is made soluble where irradiated, thereby forming an image. A technique similar thereto has been disclosed in WO97/39384.
As mentioned above, there also exists a method of preparing a conventional positive photosensitive lithographic printing plate by exposure with a laser. The conventional method employs a photosensitive layer in which a photo-acid-generator is blended with a photosensitive agent, and the energy from a laser light is used to bring about a chemical change of the additive which increases the solubility of the photosensitive layer. An alternative technique has been proposed in JP-A-10-268512, which corresponds to U.S. patent application Ser. No. 08/906,258, filed Aug. 5, 1997, wherein the layer forming a positive photosensitive lithographic printing plate is such that its alkali-solubility increases as a result of heat generated by irradiation with the laser light itself. In this case a chemical change is not substantially brought about when the alkali-solubility of the photosensitive layer increases. This alternative technique is highly advantageous because when a compound insensitive to ultraviolet rays (UV) is used as the solubility-suppressing agent or another additive, the photosensitive layer will be insensitive to UV, such that the operation can be carried out even under white light.
There are disadvantages to forming a positive image by exposure with laser and causing a chemical change. The time of irradiating one point on the photosensitive layer with the laser light is extremely short, and accordingly, photosensitivity is the problem.
To accelerate the chemical change and to increase the photosensitivity, irradiation with a short wavelength light having a high energy, is preferred. However, this approach presents a problem because laser generators having a short wavelength coupled with high power are not readily available.
Further, when the chemical change is brought about by heat converted from the light energy, problems occur because the irradiation time is extremely short. If the added amount the photo-thermal conversion material is increased to increase the amount of thermal conversion, the light is absorbed around the surface of the photosensitive layer, but in the inner layer, the light declines, whereby the reactivity will be extremely poor. As a result, the dissolution speed of the layer will decrease toward the lower layer part of the film, such that the dissolution time of the entire irradiated portion will no longer be accelerated, and there will be a limitation in the increase of the sensitivity, such being problematic.
One way of overcoming this problem is to make the photosensitive layer thin. However, when the photosensitive layer is of the thermal conversion type, if the layer is made too thin, the amount of thermal diffusion to the support increases, and the sensitivity decreases.
Further, when a large amount of solubility-suppressing agent is used, or a strong solubility-suppressing agent is used, the alkali solubility of the entire photosensitive layer decreases, whereby it becomes necessary to make the irradiation energy of laser light high and, accordingly, the exposure time tends to be long, which is disadvantageous. Further, in the inner layer, as the light declines, the reactivity will be extremely poor, and as a result, the dissolution speed will decrease toward the lower layer part of the film, whereby the dissolution time of the entire irradiated portion will no longer be accelerated, and there will be a limitation in the increase of the sensitivity.
Conversely, when the amount of the solubility-suppressing agent is small, or a weak solubility-suppressing agent is used, the strength of the entire photosensitive layer tends to be low, and there are problems in the printing resistance and the preservation property.
Further, the preservation property, the printing resistance and the chemical resistance are significantly influenced by deterioration with age or the preservation conditions. Particularly, e.g., when the molecular weight of binder components in a photosensitive material is changed to improve the chemical resistance or the printing resistance, the alkali solubility will be poor, whereby the development property will deteriorate. As mentioned above, the chemical resistance, the preservation property and the printing resistance are always in an antinomic relation to performance characteristics such as sensitivity or the development property, and the performance characteristics required for the positive photosensitive lithographic printing plate are not satisfied.
The object of the present invention is to provide a positive photosensitive lithographic printing plate which can be exposed by scanning with a laser light beam, which plate is characterized by high sensitivity and a good preservation property, an excellent solubility of the exposed portion of the photosensitive layer at the time of development, the ability to develop a clear image, and excellent printing resistance and chemical resistance. The invention also includes a method for producing the plate and a method for forming a positive image employing the positive photosensitive lithographic printing plate of the invention.
Another object of the present invention is to provide a positive photosensitive lithographic printing plate capable of forming an image by using a long wavelength laser light having a wavelength range of from 600 nm to 1,300 nm.
Another object of the present invention is to provide a positive photosensitive lithographic printing plate capable of forming an image by using a laser light, and having a uniform developing property at the entire plate.
Another object of the present invention is to provide a method for producing said positive photosensitive lithographic printing plate with a low treatment cost per plate, by downsizing the space for treatment.
Further, another object of the present invention is to provide a method for producing a positive photosensitive lithographic printing plate having uniform characteristics of the photosensitive layer and capable of printing a beautiful image.
The present invention has been made as a result of extensive studies to achieve the above objects. More particularly, it is the object of the present invention to provide:
(1) A positive photosensitive lithographic printing plate which comprises a photosensitive material containing a photo-thermal conversion material having an absorption band within a wavelength range of from 600 nm to 1,300 nm and an alkali-soluble resin, wherein the dissolution rate of said photosensitive material, in unexposed condition, in an alkali developer increases from the upper or surface part of said photosensitive material toward the lower part of said photosensitive material,
(2) A method for producing a positive photosensitive lithographic printing plate, which comprises coating a photosensitive composition containing a photo-thermal conversion material having an absorption band within a wavelength range of from 600 nm to 1,300 nm and an alkali-soluble resin, on a support to form a layer of photosensitive material, and diffusing a compound having a polar group from the surface of the photosensitive material,
(3) A method for producing a positive photosensitive lithographic printing plate, which comprises coating a photosensitive composition containing a photo-thermal conversion material having an absorption band within a wavelength range of from 600 nm to 1,300 nm and an alkali-soluble resin, on a support to form a photosensitive material, overlaying said photosensitive material with a protective material containing moisture, and keeping the overlaid, photosensitive layer under heating conditions,
(4) A method for producing a positive photosensitive lithographic printing plate, wherein a photosensitive composition comprising a photo-thermal conversion material having an absorption band within a wavelength range of from 600 nm to 1,300 nm and an alkali-soluble resin, is coated on a support to form a layer of photosensitive material, which is then overlaid with a protective material to form a predetermined size of lithographic printing plate, a plurality of such lithographic printing plates are piled one on another, a heat-insulating material is applied to almost entire top and bottom surfaces thereof, and the pile is kept under heating under such a state,
(5) A method for producing a positive photosensitive lithographic printing plate, wherein a photosensitive composition containing a photo-thermal conversion material having an absorption band within a wavelength range of from 600 nm to 1,300 nm and an alkali-soluble resin, is coated on a support to form a layer of photosensitive material, and a drying process of drying at a temperature within a range of from 20xc2x0 C. to 100xc2x0 C. for a predetermined time is carried out, prior to diffusion from the surface of the photosensitive material, and
(6) A method for forming a positive image, which comprises scanning and irradiating the positive photosensitive lithographic printing plate as defined in (1), with a laser light having a wavelength range of from 600 to 1,300 nm, to project an image for exposure, followed by development with an alkali developer.